The acquisition would unite Transco's 10,000-mile natural gas transmission system, the Transcontinental Gas Pipe Line, which extends from the Gulf of Mexico across the Deep South and up to New England, with Texas Gas's 6,000-mile distribution network, centered in the South and the Middle Atlantic region.

The deal, announced by both companies today, is expected to close by early spring, at which time it may be subject to certain adjustments. Restructuring at CSX

CSX, which bought Texas Gas and other energy assets not included in the sale to Transco in 1983 for $1 billion, is selling the pipeline company as part of a corporate restructuring. The company said in a statement that it also expected to realize a $75 million cash payment from its Texas Gas subsidiary before the closing of the sale this spring.

Transco also announced that the CNG Transmission Corporation, a subsidiary of the Pittsburgh-based Consolidated Natural Gas Company, had agreed to provide pipeline capacity needed to connect the Texas Gas system with the New York-New Jersey area, where Transco is already a leading supplier.

Consolidated, a utility holding company, also has an option to purchase half of Texas Gas. The option must be recognized within 30 days of the CSX sale's closing. Consolidated is considered by Wall Street to be one of the strongest gas companies in the nation and well positioned to buy one-half of Texas Gas. 'It Makes a Lot of Sense'

Analysts hailed Transco's decision to expand the Transcontinental Gas Pipe Line, saying that the Texas Gas system provides a necessary link between the strong Southwestern assets of the company and its fastest-growing markets.

''From a strategic point of view, it makes a lot of sense,'' said Lawrence A. Crowley, gas pipeline analyst for Rauscher Pierce Refsnes Inc. in Houston. ''Basically, they have done an end run around their capacity constraints, because Transcontinental does have capacity in the Southwest and Louisiana and constraints east of Tennessee.''

Roger J. Khlopin, gas analyst for Smith Barney, Harris Upham & Company, said the acquisition would also give Transco a dominant position in what the experts believe will be the industry's most lucrative regional market during the next decade.

''The delivered price of gas is higher in the Northeast,'' Mr. Khlopin said. ''It's a high-value product because supplies are so tight.''

Gas utilization in the Northeast, where electricity and coal have traditionally enjoyed high popularity, is expected to grow by one-third within the next decade, because of its relatively low cost and cleanliness as a fuel, Mr. Khlopin said. Price Includes Healthy Premium

Transco paid a healthy premium to buy the CSX units, approximately 10 times the company's earnings multiple at a time when pipeline companies have been selling for about six times their earnings, Mr. Crowley said, adding that he considered the price well justified for strategic reasons. Transco officials said that the acquisition of Texas Gas would not dilute the company's earnings in 1989.

Texas Gas, based in Owensboro, Ky., has been considered to be a well-run, trouble-free pipeline company in what has been a troubled interstate industry.

On the New York Stock Exchange today, CSX stock closed at $31.50 a share, down 25 cents, while Transco Energy ended at $34.625 a share, up 25 cents.